A conventional system is present that inserts dummy light formed of spontaneous emission light into an unused channel of a wavelength division multiplexing (WDM) system. In this system, the wavelength of the dummy light is selected based on distortion of a gain spectrum of the WDM system (Published Japanese-Translation of PCT Application, Publication No. 2008-510388).
Conventionally, all the wavelengths used in a system may be divided by a constant wavelength band into multiple sections called sub-bands and for each sub-band, dummy light of the same wavelength band as the sub-band wavelength band is inserted. In the following description, the dummy light of the same wavelength band as a sub-band wavelength band will sometimes be referred to as sub-band dummy light.
A sub-band includes multiple channels, i.e., wavelengths, and the transmission power of the sub-band dummy light is greater than the transmission power of the signal light of one channel. Therefore, the width of a sub-band is determined such that the transmission power of the sub-band dummy light becomes equivalent to the total transmission power of the signal light of all the channels included in the sub-band.
Therefore, if signal light is newly added to some channels of a sub-band from which sub-band dummy light is removed, the transmission power lost by removal of the sub-band dummy light cannot be compensated by the transmission power of the added signal light alone. In this case, to compensate for the shortage of the transmission power, the transmission level of sub-band dummy light of another sub-band is adjusted in some cases. However, the sub-band dummy light has a wavelength band that is wider than the signal light and therefore, causes a problem in that the transmission level of the sub-band dummy light cannot be finely adjusted.
If sub-band dummy light is inserted into a sub-band from which signal light has been removed or if an optical cable is additionally connected for restoration of a transmission path where a failure occurs, the transmission level of sub-band dummy light of another sub-band may be adjusted. Therefore, the same problem occurs in these cases.
Particularly, in a sub-band in a nonlinear state in which a nonlinear effect occurs, signal light characteristics at an opposing station deteriorate even when the transmission level at the transmission end is increased. Further, signal characteristics become unstable and a slight variation in the transmission level may cause abrupt fluctuations or deterioration of the signal characteristics. Therefore, although it is desirable for a sub-band in the nonlinear state not to change the transmission level of the sub-band dummy light, the transmission level is changed for improvement in the characteristics in some cases.
If interference occurs between signal light and dummy light due to a phenomenon called optical four-wave mixing, which is a main cause of the nonlinear effect, a variation in transmission level of the dummy light affects the signal light. If the wavelength band of the dummy light is the same as the wavelength band of the sub-band, the power ratio of the dummy light to the signal light becomes large and therefore, if the transmission level of the sub-band dummy light cannot be finely adjusted, the signal light is significantly affected.
Instead of inserting the sub-band dummy light, dummy light having the same wavelength as signal light can be inserted for each channel. In the following description, dummy light having the same wavelength as signal light will sometimes referred to as channel dummy light. If a distinction between the sub-band dummy light and the channel dummy light is not necessary, the light will sometimes be referred to as simply dummy light.
However, if the channel dummy light is inserted, since the number of channels into which the dummy light is the product of the number of sub-bands multiplied by the number of channels per sub-band, the number of channels into which the dummy light is inserted becomes larger than in a case of inserting the sub-band dummy light. Therefore, at the time of initial introduction of an optical transmission apparatus or at the time of addition or removal of signal light, when the transmission levels of the signal light and the dummy light are adjusted in the transmission-side optical transmission apparatus so as to optimize the reception-side signal light characteristics of the opposing station, time and effort are problematically required.